1. Field of the Invention
The present invention relates to a color thermal printing method, and more particularly to an improved color thermal printing method preventing neighboring thermosensitive coloring layers from being colored at the same time.
2. Description Related to the Prior Art
A thermosensitive color recording material has been known, e.g. from U.S. Pat No. 4,734,704, which can directly print a full-color image using a thermal head without using a color ink ribbon. A commonly assigned Japanese patent application, laid open to the public as JP-A 3-288688, discloses another type of a thermosensitive color recording material 7 illustrated in FIG. 10. This material has cyan, magenta, and yellow thermosensitive coloring layers 3, 4, and 5, and a protective layer 6 formed on a support 2 in this order. In this type of the recording material 7, the heat sensitivity of the uppermost yellow coloring layer 5 is highest, and that of the undermost cyan coloring layer 3 is lowest (see FIG. 2).
The cyan coloring layer 3 contains as its main components an electron-donor type dye precursor and an electron-acceptor type compound, and forms a cyan dye when heated. The magenta coloring layer 4 contains a diazonium salt compound having a maximum absorption wavelength of 360.+-.20 nm, for example 365 nm, and a coupler which forms a magenta dye when it is thermally reacted with the diazonium salt compound. When an ultraviolet ray of 365 nm is applied to the magenta coloring layer 4 after thermal printing, the diazonium salt compound is discomposed photochemically and loses a coloring ability. The yellow coloring layer 5 contains a diazonium salt compound having a maximum absorption wavelength of 420.+-.20 nm, for example 420 nm, and a coupler which forms a yellow dye when it is thermally reacted with the diazonium salt compound. When a near-ultraviolet ray of 420 nm is applied to the yellow coloring layer 5, it is fixed and loses a coloring capacity.
When recording a full-color image on the above-described recording material 7, a thermal head having a plurality of heating elements arranged in a line is used. First, the yellow coloring layer 5, disposed to be the uppermost of the coloring layers, is applied to thermal recording, in course of relative movement between the thermal head and the recording material 7. During the thermal recording, each heating element of the thermal head is supplied with a bias pulse having a relatively large width for heating the recording material 7 nearly up to the coloring temperature and then a number of image pulses having a smaller width for changing the power-on time depending upon the pixel optical density of an original image and forming color pixels having a desired optical density. This method of driving heating elements is described, for example, in commonly assigned Japanese patent application laid open to the public as JP-A 3-221468. After thermally recording a yellow image, a near-ultraviolet ray of 420 nm is applied to optically fix the yellow image. Next, the magenta coloring layer 4, or the second uppermost layer, is applied to thermal recording by using a higher heat energy than that applied for the yellow coloring layer 5. Thereafter, the magenta image is optically fixed by exposure to an ultraviolet ray of 365 nm. Lastly, the cyan coloring layer 3, or the undermost layer, is applied to thermal recording by using a highest heat energy.
The recording material 7 has intermediate layers formed between the coloring layers, though they are not shown in FIG. 10. When the respective intermediate layer is increased in thickness, the overlap between coloring characteristic curves can be avoided, even through there is a decrease in the heat sensitivity posing a problem in practical use. Such a recording material having no overlap of the characteristic curves has been proposed, for example, in JP-A 4-28585. With this recording material, first, the yellow coloring layer (the uppermost) is heated by a thermal head, the heat allowing only the yellow coloring layer to develop color, to react the diazonium salt compound contained in the layer with the coupler and form a yellow dye. After the yellow coloring layer is heated and fixed, the magenta coloring layer (the second uppermost) is heated by the thermal head, the heat allowing only the magenta coloring layer to develop color and not allowing the cyan coloring layer (the undermost) to develop color. After the magenta coloring layer is fixed, the cyan coloring layer is heated to develop cyan color. The half tone image for yellow, magenta, and cyan can be independently recorded without color mixture by driving the thermal head under the following conditions:
Thermal head: printing energy of 0.5 W/dot (manufactured by Kyocera Corporation);
Pixel density: 8 lines/mm, namely 16 dots/mm;
Thermal head driving pulse: having a constant voltage and a power-on time changing by 0.2 ms pitch depending on the tone level:
Yellow: 0.4 to 2.0 ms; PA1 Magenta: 2.4 to 4.0 ms; and PA1 Cyan: 4.4 to 6.0 ms.
To use different recording materials in which coloring characteristic curves of coloring layers are overlapped between the colors, it is required not only to record density of each color as high as desired but also suppressing development of the color of which the characteristic curve is overlapped with that of the color to be developed. To be precise, the high density range of the yellow coloring layer 5 (in a zone EA in the graph of FIG. 2) overlaps with the low density range of the magenta coloring layer 4. Therefore, when a high density image for yellow is recorded, the magenta coloring layer 4 develops color by the heat energy applied for coloring the yellow image to cause color mixture of magenta with yellow as illustrated in FIG. 2, which results in failure in reproducing the color hue with fidelity. The high density range of the magenta coloring layer 4 (in a zone EB in the graph of FIG. 2) overlaps with the low density range of the cyan coloring layer 3. When a high density for magenta is recorded, the cyan coloring layer 3 develops color by the heat for coloring magenta to cause color mixture of cyan with magenta, which results in failure in color reproduction.
In view of this, for the thermal recording of the yellow and magenta coloring layers 5 and 4, the heat energy in use could be limited in a predetermined smaller range than the smallest energy which develops color of a coloring layer under-lying the relevant coloring layer. However, this improvement in turn would make it impossible to reproducing high density in images. There would take place a further problem in that, as illustrated in FIG. 11, a portion 8 of a coloring layer within the one pixel 9, as colored by a single heat element as a color dot, would be conspicuously smaller than the pixel 9, and apparently surrounded by blank ground.